Pure necessity. After Hitler came to power, he knew that he needed allies. Also, Japan had its own imperial ambitions. However, for centuries, nation states that had been deemed too powerful or perceived as a threat were met with a coalition or some form of an alliance to ensure that said nation would not become too powerful and hurt the members of the alliance/coalition. Some good examples of this are Napoleonic France, pretty much the whole alliance structures leading to WWI, Sweden in the Great Northern War, and even Saddam's invasion of Kuwait. Germany especially needed a friend in the fight to come; Italy being the first. Hitler always knew that Germany would need to militarily conquer other countries in Europe. Likewise, Japan is completely isolated. Japan's hope was to amass a large empire that would include much of Asia and Oceania. But for them to just invade their desired territory would most likely be met with some other foreign power's intervention. Though Hitler believed the "German Race" to be the superior race on the planet and wanted total world domination, Germany could not fight everyone. The same principle goes with Japan.

To make matters worse, Japan, like the rest of the Axis Powers, felt that the result of the First World War and the subsequent Treaty of Versailles were particularly harmful to each respective Axis Power.